Nuclear magnetic resonance spectrometer

ABSTRACT

Method and apparatus for measuring gyromagnetic resonances over the nuclear magnetic resonance spectrum wherein the driving ratio frequency magnetic field or the unidirectional polarizing magnetic field is modulated at two or more different frequencies creating different resonance regions on the spectrum which are observed substantially simultaneously.

United States Patent Takeuchi et al.

NUCLEAR MAGNETIC RESONANCE SPECTROMETER Inventors: Makoto Takeuchi;Kazuo Nakagawa; Teruo Miyamae; Terubumi Kase, all of Tokyo, JapanAssignee: Nihon Denshi Kabushiki Kaisha, Tokyo,

Japan Filed: Dec. 29, 1969 Appl. No.: 888,374

Foreign Application Priority Data Nov. 27, 1969 Japan ..44/95300 U.S. CL..324/0.5 R

Int. Cl. ..G0ln 27/78 Field of Search ..324/0.5 A, 0.5 AC, 0.5 G

[ Feb. 29, 1972 [56] References Cited UNITED STATES PATENTS 3,304,4922/1967 Glarum ..324/0.5 A 3,487,293 12/1969 Toshimasa ..324/0.5 AC

Primary Examiner- MichaelJ Lynch Attorney-WebbBirrden, Robinson & Webb[5 7] ABSTRACT Method and apparatus for measuring gyromagneticresonances over the nuclear magnetic resonance spectrum wherein thedriving ratio frequency magnetic field or the unidirectional polarizingmagnetic field is modulated at two or more different frequenciescreating different resonance regions on the spectrum which are observedsubstantially simultaneously.

6 Claims, 9 Drawing Figures RF R PUFIER CIRCIII T MIXER PULSE GENERATORPATENTEDFEBZS m2 3,646,429

SHEET 2 OF 4 fig. 5

fig- 5 WW a 5 PATENTEDFEBZQ I972 3,646,429 SHEET 3 OF 4 59/455 ,4 002mm? ao/warm L COMPUTER RECORDER PHASE A D j J DE 7H TOR CONVERTER 242/ NUCLEAR MAGNETIC RESONANCE SPECTROME'IER In recent years attemptshave been made to observe with high resolution the nuclear magneticresonance spectra of nuclei other than the proton or fluorine nucleus,for example C, Si, N and "B. These nuclei have extremely small naturalabundances and, therefore, produce very weak resonance signals.

Among the mentioned nuclei, C and N are of special interest inconnection with the chemical structure of molecules and chemicalreactions, etc. -"C has drawn rather special attention due to the wideapplicability.

Consider the observation of C in which all the proton nuclei aredecoupled. Although the natural abundance of the C nucleus is only 1.1percent and its signal intensity a mere 1.59 percent (approximately) ascompared with that of the proton, none the less, good results may beobtained when observing C signals in the high magnetic field whilecarrying out complete proton decoupling, due to collapsing the multipletand the nuclear Overhauser Effect between the protons and C. The linewidth of the proton decoupled spectrum is extremely narrow and the rangeof the chemical shift is larger than that of proton by one order.Consequently, the spectra hardly ever overlap.

When the spectrum line is very sharp, however, the magnetic field mustbe swept very slowly in order to observe the resonance spectraaccurately. If, for example, the field is swept rapidly, the spectrawill be distorted resulting in their appearing as broad line spectra. Atthe same time, if the chemical shift is extremely large and the chemicalshift region is swept slowly, observation will become quite timeconsummg.

Generally, when carrying out C magnetic resonance observation, thecomputer time averaging technique is utilized to improve S/N (signal tonoise) ratio when sweeping the field slowly and thereby improve thereproduction fidelity of the resonance spectrum. However, it takes quitea long time to improve the S/N ratio for the reason that the S/N ratiois proportional to the square root of the number of measurements.

It is true, of course, that the above shortcoming can be overcome byemploying two observing systems instead of one. In this case, the twosystems are independently arranged so that each system simultaneouslyobserves half the chemical shift range, thus reducing the sweep time byhalf. In other words, by means of this twin system arrangement, therequired S/N ratio can be obtained in half the time necessary for theconventional arrangement, since each output signal is independently andseparately accumulated by a computer.

However, the utilization of two systems tends to make the apparatusquite complex, not to mention the added cost of such an apparatus.However, it has been found to be extremely difficult to balance the twosystems perfectly due to the slight differences in the operatingconditions of the incorporated component parts.

According to this invention, there is provided a novel method andapparatus for reducing the observation time by using a time sharingmethod. Another feature of this invention is the provision of a novelmethod and apparatus for enlarging spectra having small magnitude. Stillanother feature of this invention is the provision of a novel method andapparatus for improving the S/N (signal to noise) ratio by utilizing thenoise time averaging method.

Briefly, according to this invention, a high resolution magneticresonance spectrometer, effective for observing nuclei having extremelysmall natural abundances, utilizes a timesharing technique in which twoor more different modulating frequencies are applied to the drivingradiofrequency magnetic field or the unidirectional polarizing magneticfield to create two different resonance regions on the gyromagneticspectrum which may be observed substantially simultaneously. Accordingto one embodiment of this invention the two or more modulatingfrequencies are alternately applied and their respective resonanceregions observed during the time of application. According to anotherembodiment, the two or more modulating frequencies are simultaneouslyapplied and the resonance signals in the respective resonance regionsare alternately observed by alternately adding selected localfrequencies to the signals such that only the signal from resonanceregions passes a filter. Other features and advantages will become morereadily apparent by reading through the following detailed descriptionof the present invention in conjunction with the accompanying drawingsin which:

FIG. 1 is a block schematic showing one embodiment of the presentinvention wherein a magnetic field modulated gyromagnetic apparatus isused;

FIG. 2 shows the two modulation frequency modes used in the embodimentaccording to FIG. 1;

FIGS. 3 and 4 are graphical illustrations of the spectra used forexplaining the present invention;

FIG. 5 is a graphical illustration of the spectrum used for explainingthe modified embodiment according to FIG. 6;

FIG. 6 shows a partially modified embodiment of the present invention;

FIG. 7 shows the spectra obtained by the apparatus according to thepresent invention;

FIG. 8 shows another embodiment of this invention; and

FIG. 9 shows spectra obtained by using the embodiment according to FIG.8.

Referring now to FIG. 1, a test sample 1 is placed in a unidirectionalmagnetic field produced by an electromagnet 2, the said magnet beingexcited by an excitation source (not shown).

An RF coil 3 perpendicular to the unidirectional magnetic field suppliesa radiofrequency driving magnetic field to the sample I, the necessaryradiofrequency being provided by an RF transmitter 4 via an attenuator 5and a bridge detector 6.

A pair of modulation coils 7a and 7b, arranged coaxially with respect tothe unidirectional magnetic field, are alternately supplied with, forexample, a 4 kHz. (m and a 5 kHz. (m modulating current by means ofaudiofrequency oscillators 8 and 9 through gate circuits 10 and 11. Apulse generator 12 supplies pulsed outputs to the said gate circuits 10and II and, since the said outputs are in antiphase, the modulationcurrents (m and 0, are alternately applied to the modulation coils 7aand 7b as shown in FIG. 2.

When the unidirectional magnetic field is swept by means of a sweep coil(not shown), the nuclear magnetic resonance signals 21rfw and 21rfw(where f, is the RF transmitter frequency, for example, 60 mc.) of thesample I are produced and fed into an RF amplifier 13 via the bridgedetector 6. The amplified resonance signals are then fed into an RFmixer 14 (heterodyne detector) to which the radiofrequency]; issimultaneously applied from the RF transmitter 4 as a reference signal.As a result, after suitable filtering, output signals having only twocomponents, namely m and m are selected. The said output signals arethen split and fed into gate circuits I5 and I6 to which pulses from thepulse generator R2 are applied. The gates are adjusted so that when thetime shared modulation frequency m is applied to the coils 7a and 7b thegate circuit 15 is opened and gate l6 is closed; conversely, when thetime shared modulation frequency m is applied to the coils 7a and 7b,gate 16 is opened and gate 15 is closed.

The output signals from the gates are then fed into phase detectors l9and 20, after being amplified by AF amplifiers I7 and 18, respectively.At the same time, modulation frequencies w, and m are applied from theaudiofrequency oscillators 8 and 9 to the phase detectors I9 and 20, asreference signals, respectively.

Thus, by setting the phase of the reference signal either at 0 or 90 bymeans of a phase shifter (not shown), either the dispersion mode orabsorption mode can be optionally selected and recorded on either atwo-pen recorder or two separate one-pen recorders. In this way, thespectra of C, for example, can be obtained.

The operation of the apparatus according to the invention is set forthbelow in reference to the embodiment diagramed in FIG. I.

mums 0414 METHOD ONE According to this method the modulation range isdivided into two equal parts or more according to the number ofmodulation frequencies. FIG. 3 shows the resonance spectrum lines ofethylbenzene (C I-l CH CI-I to which tetramethyl silane (TMS) has beenadded for reference. The spectrum A- C is divided into two equal parts(A-B) (B-C) and the magnetic field or frequency corresponding to eachpart is swept simultaneously. That is to say, the magnetic field orfrequency corresponding to (A-B) is swept by to and the magnetic fieldor frequency corresponding to (B-C) is swept by m The resultantresonance spectrum lines of the two halves of the spectrum, i.e., (A-B)and (-C), appear as shown in FIGS. 4(a) and (b) respectively.

In accordance with this method, therefore, the entire spectrum isobserved in half the time necessary for conventional sweep. Moreover, byusing more than two modulation frequencies, the measuring time can bereduced. Again, since when observing unknown samples the position of thespectra are unknown, it is very advantageous to shorten the sweep timeby applying the above-mentioned method.

METHOD TWO According to this method two modulation frequencies are setvery close to each other, but not so close as to cause interactionbetween the two phase detectors. By making the difference Am betweenmodulation frequencies m and m comparatively small, 01,, resonates atspectrum A and m resonates at spectrum B, as shown in FIG. 5. As aresult, each resonance signal is fed into the phase detectors [9 and2.0, respectively, as shown in FIG. 6. At the same time, m and m areapplied to the phase detectors as reference signals. Each resonancefrequency component is introduced into a computer 24 via A-D (analog todigital) converters 22 and 23. Once in the computer, the output from theA-D converter 23 is shifted by Am and thereafter added to the outputfrom the A-D converter 22. Thus, the accumulated signals are indicatedon the recorder 21.

By means of this method, the signal intensity is twice that of theconventional method and the noise component becomes \fi sothat theSjliratio increase to V5. This method is very advantageous because the S/Nratio can be improved by setting the modulation frequencies m and m veryclose together, even though the measuring time is almost equal to thatof the conventional method using one modulation frequency. Moreover, itgoes without saying that to use more than two modulation frequencieswould be even more effective for improving the S/N ratio.

METHOD THREE According to this method, simultaneous ordinal andpartially magnified measurements are made. When it is desired to observethe whole spectrum and a desired part of the spectrum simultaneously,the modulation frequencies (0,, and m are set so that to, sweeps thewhole spectrum and m sweeps the said desired part of the spectrum withthe same measuring time, that is, at different sweep rates. Inaccordance with this method, the spectra are obtained as shown in FIG. 7where FIG. 7(a) shows the spectra of the entire spectrum and FIG. 7(b)shows the magnified spectra of CH;,.

FIG. 8 which utilizes the same units as FIG. 1, shows the block diagramof another embodiment of the gyromagnetic resonance spectrometer systemutilizing the time sharing technique designed in accordance with thepresent invention.

METHOD FOUR This invention is applicable to the invention described inthe US. Pat. No. 3,462,676, filed Nov. 29, 1966, describing a Method ForProducing Gyromagnetic Resonance. This patent disclosed a method foreffectively obtaining and measuring the resonance of nuclei such as C,p, F and protons which are adjacent to paramagnetic metals, all the saidnuclei being characterized by their larger chemical shifts. The patentutilizes the nuclear resonance single sideband method (NSS System) inorder to measure the resonance signal of a sample having a largechemical shift.

In the embodiment as shown in FIG. 8, modulation frequen cies m and (u(for example, 4 kHz. and 5 KHz.) are simultaneously and continuouslyapplied to the modulation coils 7a and 7b. The RF driving field isselected so as to give the optimum resonance condition at the firstsideband (n=l) in order to saturate the main band at n=0. The resonancesignals of the sample 1 are produced by sweeping the unidirectionalmagnetic field and are fed into an RF amplifier 13 through a bridgedetector 6. The output signals from the RF amplifier are transmitted toa mixer 14 to which different radiofrequencies are alternately appliedfrom local oscillators 25 and 26 through gate circuits l0 and II and amixer 27. The output signals of the local oscillators are, for example,459 kHz. and 460 kHz. Since the outputs of pulse generator 12 applied tothe said gate circuits are in antiphase, the outputs from the localoscillators 2S and 26 are alternately applied to the mixer 27 to whichthe RF driving field f, (60 me.) is simultaneously supplied from the RFtransmitter 4. The output signals f459 kHz. and f t460 kI-Iz. from themixer 27 are then alternately fed into the mixer 14. Mixer l4 mixes thesignals fed from the RF amplifier 13 with the fgt459 kHz. and fa -460kHz. signals resulting in output signals having frequency components 459KHzi i kHz. and 460 kHLiS kI-Iz. These components are then transmittedto an intermediate frequency amplifier 28 provided with a crystal filterhaving a very narrow band-pass width. 455 kHz. is now set as the centerfrequency. The output signals, 459-4 kHz. and 460-5 kI-Iz., are selectedby the crystal filter and then fed into gate circuits l5 and 16 to whichpulses are supplied from the pulse generator 12. These gate 5 circuitsoperate in the same way as the gate circuits l0 and ll.

The outputs passed through the gates 15 and 16 are fed into mixers 29and 30 to which the local oscillator signals 45 9 kHz. and 460 kHz. areapplied as reference signals, respectively. The output signals, eachhaving only one component 4 kHz. and 5 kHz., are selected from the saidmixer. 29 and 30, and are then introduced into phase detectors l9 and 20via audiofrequency amplifiers l7 and 18, respectively. At the same time,the audiofrequency oscillators 8 and 9 apply reference signals m and mto the said phase detectors respectively.

Thus, by setting the reference signals to 0 or by means of a phaseshifter (not shown) either the dispersion mode or absorption mode can beoptionally selected and recorded. The resultant spectra are shown inFIG. 9 (a) and (b).

With the above embodiment utilizing the NSS System, the differentresonance regions can be observed simultaneously by applying two or moredifferent modulation frequencies and by alternately and continuouslyapplying two or more different local oscillator frequencies,corresponding to the said two or more different modulation frequencies,to a mixer.

Moreover, in order to improve the SIN ratio, the AD converters andcomputer can be arranged as shown in FIG. 6.

Although the present invention describes the utilization of a magneticfield modulation-type gyromagnetic resonance apparatus, other types ofgyromagnetic resonance apparatus, for example, the radiofrequencymagnetic field modulation-type apparatus may be utilized in lieu. Inother words, it is not limited to one type of apparatus.

This invention may be also used with either an internal or 5 externalcontrol sample system.

The invention as described above can also be applied to a protonmagnetic resonance spectrometer without any change or modification.

In the specification and claims, by substantially simultaneously we meanintermittently over short intervals as determined by the frequency ofthe pulse generator.

While we have shown and described preferred embodiments of ourinvention, other modified embodiments within the scope of the appendedclaims may be applied.

We claim:

l. A nuclear magnetic resonance spectrometer for measuring gyromagneticresonance in a sample comprising:

a. means for positioning the sample in a unidirectional magnetic field,

b. means for producing the unidirectional magnetic field for polarizingsaid sample,

c. means for applying a driving radiofrequency magnetic field to saidsample at right angles to said unidirectional magnetic field so as toproduce gyromagnetic resonance in said sample,

d. means for modulating one of said unidirectional or radiofrequencymagnetic fields applied to the sample with at least two differentfrequencies,

e. means for substantially simultaneously detecting the respectiveresonance signals corresponding to said different modulationfrequencies,

f. gate circuits for alternately applying said different modulationfrequencies to said sample,

g. means in synchronism with said gate circuits for alternatelyselecting one resonance frequency component from the respectiveresonance signals based on the different modulation frequencies, and

h. means for recording said respective resonance signals.

2. A nuclear magnetic resonance spectrometer for measuring gyromagneticresonance in a sample according to claim 1 comprising:

means for accumulating the resonance signals, the said accumulatingmeans shifting one of the resonance signals by only the differencebetween the modulation frequencies and then accumulating the shiftedresonance signal and the remaining resonance signal, whereby the signalto noise ratio is improved.

3. A nuclear magnetic resonance spectrometer for measuring gyromagneticresonance in a sample comprising:

a. a sample positioned in a unidirectional magnetic field,

b. means for producing the unidirectional magnetic field for polarizingsaid sample,

c. means for applying a driving radiofrequency magnetic field so as toproduce gyromagnetic resonance in said sample,

d. means for simultaneously modulating said unidirectional magneticfield with at least two different modulation frequencies,

e. means for detecting the respective resonance signals corresponding tosaid different modulation frequencies,

f. local oscillators for producing at least two differentradiofrequencies corresponding to the said at least two differentmodulation frequencies,

g. first gate circuits for alternately and continuously passing saiddifferent radiofrequencies,

h. means for mixing resonance signals in said sample with thealternately and continuously passed different radiofrequencies, in orderto obtain the intermediate frequency resonance signal components,

i. a means for filtering these said intermediate frequency resonancesignal components in such a manner as to select only one sidebandfrequency component at a time corresponding to each respectivemodulation frequency,

j. second gate circuits for alternately and continuously passing thesaid band frequency components, k. means for heterodyne detectionwherein the respective output signals from the second gate circuits andradiofrequencies from the local oscillators are mixed and heterodynedetected so as to simultaneously detect only the respectiveaudiofrequency resonance signals, and 1. means for recording saidrespective resonance signals. 5 4. A nuclear magnetic resonancespectrometer for measuring gyromagnetic resonance in a sample accordingto claim 3 comprising:

a. means for accumulating the resonance signals, the said accumulatingmeans shifting one of the resonance signals by only the differencebetween the modulation frequencies and thenaccumulating the shiftedresonance signal and the remaining resonance signal in order to improvethe S/N ratio, and

b. means for recording the resonance signals.

5. A method for measuring gyromagnetic resonance on the nuclear magneticresonance spectrum comprising the steps for:

a. applying a unidirectional polarizing magnetic field to the sample,

b. applying a driving radiofrequency magnetic field to the sample atright angles to said unidirectional magnetic field,

c. simultaneously modulating said unidirectional magnetic field with atleast two different modulating audiofrequencies, thereby producing atleast two resonance regions on the gyromagnetic spectrum in whichresonance signals occur,

d. detecting signals from the sample when it is at resonance in anyresonance region,

e. alternately mixing the resonance signals with local frequency signalsselected to permit mixed signals corresponding to any one resonancesignal to pass a narrow band filter,

f. passing the mixed signals through a narrow band filter,

g. mixing the filtered signal with the local frequency signal whichenables it to pass the filter to obtain an audiofrequency signal,

h. passing the signal to a phase detector to which the respective audiomodulating frequency is simultaneously fed, and

i. alternately recording the resonance in the different resonance on thegyromagnetic spectrum.

6. A method for measuring gyromagnetic resonance over the nuclearresonance spectrum in a sample comprising the steps for:

a. applying a unidirectional polarizing magnetic field to the sample,

b. applying a driving radiofrequency magnetic field to said sample atright angles to said unidirectional magnetic field,

c. modulating one of said unidirectional magnetic field orradiofrequency fields applied to said sample with at least two differentfrequencies to create two different resonance regions on thegyromagnetic spectrum, said different frequencies being alternatelyapplied,

(1. selecting one resonance frequency component from the respectiveresonance signals based on the different modulating frequencies, and

e. observing substantially simultaneously the selected resonance signalscorresponding to the different modulating frequencies and recording therespective resonance signals.

v UNITED STATES- PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3646 429 Dated February 29, 1972 Invent0r($) Makoto Takeuchi et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patentare hereby corrected as shown below:

In the Abstract Line 3 --ra.tioshould read --ra.dio--. Column 3 Line 14--(-C)-- Should read --(B-C)--. Column 4 Line 1 -l arger-- should read--large--.

Signed and sealed this 11th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. A nuclear magnetic resonance spectrometer for measuring gyromagneticresonance in a sample comprising: a. means for positioning the sample ina unidirectional magnetic field, b. means for producing theunidirectional magnetic field for polarizing said sample, c. means forapplying a driving radiofrequency magnetic field to said sample at rightangles to said unidirectional magnetic field so as to producegyromagnetic resonance in said sample, d. means for modulating one ofsaid unidirectional or radiofrequency magnetic fields applied to thesample with at least two different frequencies, e. means forsubstantially simultaneously detecting the respective resonance signalscorresponding to said different modulation frequencies, f. gate circuitsfor alternately applying said different modulation frequencies to saidsample, g. means in synchronism with said gate circuits for alternatelyselecting one resonance frequency component from the respectiveresonance signals based on the different modulation frequencies, and h.means for recording said respective resonance signals.
 2. A nuclearmagnetic resonance spectrometer for measuring gyromagnetic resonance ina sample according to claim 1 comprising: means for accumulating theresonance signals, the said accumulating means shifting one of theresonance signals by only the difference between the modulationfrequencies and then accumulating the shifted resonance signal and theremaining resonance signal, whereby the signal to noise ratio isimproved.
 3. A nuclear magnetic resonance spectrometer for measuringgyromagnetic resonance in a sample comprising: a. a sample positioned ina unidirectional magnetic field, b. means for producing thEunidirectional magnetic field for polarizing said sample, c. means forapplying a driving radiofrequency magnetic field so as to producegyromagnetic resonance in said sample, d. means for simultaneouslymodulating said unidirectional magnetic field with at least twodifferent modulation frequencies, e. means for detecting the respectiveresonance signals corresponding to said different modulationfrequencies, f. local oscillators for producing at least two differentradiofrequencies corresponding to the said at least two differentmodulation frequencies, g. first gate circuits for alternately andcontinuously passing said different radiofrequencies, h. means formixing resonance signals in said sample with the alternately andcontinuously passed different radiofrequencies, in order to obtain theintermediate frequency resonance signal components, i. a means forfiltering these said intermediate frequency resonance signal componentsin such a manner as to select only one sideband frequency component at atime corresponding to each respective modulation frequency, j. secondgate circuits for alternately and continuously passing the said bandfrequency components, k. means for heterodyne detection wherein therespective output signals from the second gate circuits andradiofrequencies from the local oscillators are mixed and heterodynedetected so as to simultaneously detect only the respectiveaudiofrequency resonance signals, and l. means for recording saidrespective resonance signals.
 4. A nuclear magnetic resonancespectrometer for measuring gyromagnetic resonance in a sample accordingto claim 3 comprising: a. means for accumulating the resonance signals,the said accumulating means shifting one of the resonance signals byonly the difference between the modulation frequencies and thenaccumulating the shifted resonance signal and the remaining resonancesignal in order to improve the S/N ratio, and b. means for recording theresonance signals.
 5. A method for measuring gyromagnetic resonance onthe nuclear magnetic resonance spectrum comprising the steps for: a.applying a unidirectional polarizing magnetic field to the sample, b.applying a driving radiofrequency magnetic field to the sample at rightangles to said unidirectional magnetic field, c. simultaneouslymodulating said unidirectional magnetic field with at least twodifferent modulating audiofrequencies, thereby producing at least tworesonance regions on the gyromagnetic spectrum in which resonancesignals occur, d. detecting signals from the sample when it is atresonance in any resonance region, e. alternately mixing the resonancesignals with local frequency signals selected to permit mixed signalscorresponding to any one resonance signal to pass a narrow band filter,f. passing the mixed signals through a narrow band filter, g. mixing thefiltered signal with the local frequency signal which enables it to passthe filter to obtain an audiofrequency signal, h. passing the signal toa phase detector to which the respective audio modulating frequency issimultaneously fed, and i. alternately recording the resonance in thedifferent resonance on the gyromagnetic spectrum.
 6. A method formeasuring gyromagnetic resonance over the nuclear resonance spectrum ina sample comprising the steps for: a. applying a unidirectionalpolarizing magnetic field to the sample, b. applying a drivingradiofrequency magnetic field to said sample at right angles to saidunidirectional magnetic field, c. modulating one of said unidirectionalmagnetic field or radiofrequency fields applied to said sample with atleast two different frequencies to create two different resonanceregions on the gyromagnetic spectrum, said different frequencies beingalternately applied, d. selecting one resonance frequency component fromthe respective resonance signals based on the different modulaTingfrequencies, and e. observing substantially simultaneously the selectedresonance signals corresponding to the different modulating frequenciesand recording the respective resonance signals.